1. Field of the Invention
The present invention relates to vibrating beam force sensors in general and to the construction of producible accurate vibrating beam force sensors in particular.
2. Description of the Prior Art
Vibrating beam force sensors (also known as resonant sensors) are well known in the art. A vibrating beam comprised of a quartz crystal is supplied with an electrical drive signal which causes the beam to resonate at a frequency dependent upon axial force applied to the beam. If the force is compressive, it decreases the vibrating frequency and, if the force is tensile, it increases the vibration frequency. The vibrating frequency can be sensed and provides an indication of the force applied. The force applied can be created by any structure, but of particular interest is a force derived from a bellows providing an indication of pressure or a force derived from a proof mass providing an indication of acceleration.
In U.S. Pat. No. 4,406,966 a structure is shown which mounts a vibrating quartz beam in the form of a double ended tuning fork (DETF) design so that forces on the supporting structure change the vibrational frequency of the tuning fork. Difficulties of such a system are that, because of the different coefficients of expansion of the different materials (the DETF being quartz and the supporting structure being metal), inaccuracies in measurement arise.
In U.S. Pat. No. 5,596,145 issued to Albert et al on Jan. 21, 1997, a monolithic resonator for a vibrating beam sensor is disclosed (the entire subject matter of the ""145 Albert patent is herein incorporated by reference). The benefit of such a monolithic resonator is that the entire structure, including the vibrating beam and associated vibrational isolator mechanism, can all be machined out of a single quartz structure. Benefits and details of such a monolithic structure are provided in the ""145 patent and are also disclosed in FIGS. 1(a) through 2(b) in the present application.
In FIG. 1 (a), a monolithic vibrating beam structure is disclosed having a mount structure 10 and a mounting hole 12 for securing the structure. In the prior art pressure sensor embodiment shown in FIGS. 1(a) through 1(c), two orthogonally arranged flexure beams 14 permit the lever arm portion 16 to pivot about pivot point 18 under the influence of bellows 20. Bellows 20 may be attached to any corresponding structure which transduces changes in fluid pressure into changes in mechanical force and applies that mechanical force to the end of lever arm 16.
In the event the pressure sensor is mounted as shown, it has been found helpful to provide a balance weight 22 which offsets the weight of the lever arm and the bellows. A vibrating beam 24 is located between the end of the lever arm portion 16 and the end of mount structure 10. It is connected to these structures respectively by isolator beams 26. In order to avoid transmitting vibrations to the lever arm and the mounting structure, isolator masses 28 are provided so as to avoid transmission of vibrating quartz beam root reactions through the isolator beams into the solid structure, thereby reducing the efficiency of resonant vibration or xe2x80x9cQ.xe2x80x9d
End view FIG. 1(c) and cross-sectional view 1(b) illustrate the different thicknesses of quartz material used including the mounting structure having thickness indicated at 30, the isolator mass having thickness 32 and the vibrating beam with thickness 34. The single vibrating beam vibrates in the plane of FIG. 1(a), which also happens to be the plane in which flexing about pivot point 18 occurs. As discussed in U.S. Pat. No. 5,596,145, the above monolithic structure can be easily created. The multi-thickness integrated structure becomes practical due to the multiple thicknesses of the various portions of the structure. This allows for a very thin structure 34 for high vibrating beam sensitivity, a thicker structure 32 for desired vibration isolation mass and a substantially thicker structure 30 to provide strength of the overall design and its mount. The application of a suitable electrical drive voltage applied to electrodes 36 (whose pattern on the vibrating beam itself is not shown for clarity of illustration) causes the structure to operate.
FIG. 2(a) also illustrates the acceleration sensor of U.S. Pat. No. 5,596,145. Similar structures to those identified in FIGS. 1(a) through l(c) are indicated with similar terms in FIGS. 2(a) and 2(b). However, unlike the pressure sensor whose flexure beams 14 are orthogonally oriented (so as to provide rotation about pivot point 18), the acceleration sensor flexure beams 38 are parallel and permit proof mass 40 to move in a direction orthogonal to the parallel flexure beams. With the mount structure rigidly mounted to a base whose acceleration is to be measured, movement of the base and consequently the mount structure 10 in the direction of arrows R will permit force to be applied to the vibrating beam where the force is proportional to the acceleration of the proof mass in an up or down direction (as shown in FIG. 2(a)). The structure of FIG. 2 would also be mounted in an evacuated and sealed housing (not shown).
As with the pressure sensor shown in FIGS. 1(a) through 1(c), the acceleration sensor shown in FIGS. 2(a) and 2(b) is a monolithic structure, in that the entire device is machined from a single piece of quartz crystal. The disadvantages of the prior art shown in FIGS. 1(a) through 2(c) are producibility and cost as a result of manufacturing limitations. The machining of the outer structure having the mounting structure thickness 30 is relatively economical, because tolerances are loose and xe2x80x9ccookie cutterxe2x80x9d methods of machining, such as ultrasonic machining, can be employed.
However, the inner structures comprising the isolator mass thickness 32, and more particularly the vibrating beam thickness 34, are much more difficult to machine. These features are not only delicate, but their tolerances must be kept relatively close. As a result, the more economical ultrasonic machining methods cannot be used and slower, more expensive methods are required. In addition, because even the thin inner structure features are still too thick, photo-etch processes such as those disclosed in U.S. Pat. No. 4,215,570 issued to EerNisse relating to the disclosed double-ended tuning fork (DETF) type vibrating beam assembly, cannot be used.
Thus, the methods used to easily and conveniently machine the thick outer structure compromises the ability to maintain the high degree of tolerance needed for proper machining of the vibrating beam structure thickness.
As a result of the above prior art difficulties, it is an object of the present invention to provide a construction for a force sensor in which an outer structure capable of being produced by conventional machining methods is combined with a second structure which is thin enough to be capable of machining by more accurate photo-etch processes.
It is an additional object of the present invention to provide an outer force carrying structure and a separate inner vibrating beam transducer structure in which each separate piece is machined in the most efficient manner.
It is a still further object of the present invention to provide a combination of outer structure and inner vibrating beam structures formed in the most economical manner and yet providing a high degree of accuracy.
The above and other objects are achieved by machining the outer structure in a conventional fashion for its thickness, i.e. ultrasonic machining, photo-etch machining or abrasive jet machining. The vibrating beam inner structure is created from a relatively thin quartz substrate with conventional xe2x80x9cphoto-etchxe2x80x9d methods of machining. The need for intermediate thickness isolator beams is avoided by using a double-ended tuning fork design (DETF). In order to ensure equal force is applied to both of the vibrating beams of the DETF design, the plane of the vibrating beam assembly (and its plane of vibration) is oriented perpendicular to the plane of flexing of the outer structure.